A thin film transistor (a TFT) including a semiconductor thin film (having a thickness of several nanometers to several hundreds nanometers) which is formed over a substrate having an insulating surface is applied to a thin film integrated circuit, a liquid crystal display device, or the like. In particular, application of a thin film transistor as a switching element provided in a pixel of a liquid crystal display device is expanding.
In a conventional technique, a thin film transistor is manufactured using a silicon semiconductor. However, in recent years, a technique for manufacturing a thin film transistor using a metal oxide having semiconductor characteristics has attracted attention. Indium oxide is a well-known material as a metal oxide, and has been used as a transparent electrode material which is necessary for a liquid crystal display device because it has high conductivity.
On the other hand, it is known that a metal oxide shows semiconductor characteristics by control of the composition of the metal oxide; typically, tungsten oxide, tin oxide, indium oxide, zinc oxide, or the like can be given as a metal oxide having semiconductor characteristics. Thin film transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics (i.e., an oxide semiconductor) are already known (Patent Documents 1 to 4, Non-Patent Document 1).
As metal oxides, multi-component oxides as well as single-component oxides are known. For example, InGaO3(ZnO)m (m is a natural number) having a homologous series is known as a multi-component oxide semiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4). In addition, it has been confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide can be used as a channel layer of a thin film transistor (Patent Document 5, Non-Patent Documents 5 and 6).